Abstract: A reflective thin film polarization retardation device which is adapted for use in active mirror laser apparatus is described. The device permits obtaining phase retardation of light, different amounts of which may be readily obtained. Retardation results from interference effects within thin film arrays within the device. A polarization rotator utilizing the invention is made up of a thin film reflective transmissive polarizer, a thin film reflector and a phase adjusting layer, which may be one or more thin film layers. The phase adjusting layer is sandwiched between the thin film polarizer and the thin film reflector. The thin films constituting the polarizer, phase adjusting layer and reflector may be successively deposited on one of the planar faces of a body of laser glass, with the films constituting the polarizer being deposited first.